1. Field of the Invention
The invention relates to the field of materials sciences and relates to a component of a ferromagnetic shape memory material which can be used, for example, for microsystems, microsensors and microactuators.
2. Discussion of Background Information
Since the discovery of magnetic shape memory alloys [Ullakko, K. et al.: Appl. Phys. Lett. 69 (13), 1966 (1996)] it has been possible to achieve large strains of up to 10% in Ni—Mn—Ga single crystals in a magnetic field by reorienting the crystal structure [Sozinov, A., et al.: Appl. Phys. Lett. 80 (10), 1746 92002)]. The magnetic field strength necessary for this purpose is less than 1 T.
This discovery led to intensive research into new compositions of Ni—Mn—Ga [Takeuchi, I. et al.: Nature Materials 2, 180 (2003)] as well as other new materials, such as Ni—Mn—In [Lavrov, A., et al.: Nature 418 (6896), 385 (2002); Kainuma, R. et al.: Nature 439, 957 (2006)]. New preparation methods, such as foam [Boonyongmaneerat, Y., et al.: Phys. Rev. Lett. 99, 247201 (2007)] or composites [Scheerbaum, N., et al.: Acta Mat. 55, 2707 (2007)] were developed. Furthermore, new application possibilities were researched, such as magnetic cooling [Krenke, T. et al.: Nature Mat. 4, 450 (2005)].
The most important magnetic shape memory materials are Ni—Mn—Ga and Fe—Pd alloys. In the case of Ni—Mn—Ga, a shortening of the material along the direction of the applied magnetic field is achieved, while Fe—Pd shows an extension under the same conditions. The maximum strain is achieved only in single crystals, however.
Magnetic shape memory alloys are first of all also conventional thermal shape memory alloys [Otsuka, K., Wayman, C. M. (ed.) Shape Memory Materials (Cambridge University Press, Cambridge 1998)].
These materials have a martensitic transition from a cubic high temperature phase, the austenite, to a low-symmetrical low temperature phase, the martensite [Bhattacharya, K. et al.: Nature 428, 55 (2004)]. The martensite phase can have, e.g., a tetragonal unit cell with two “a” axes of identical length and a short “c” axis. Through the phase transition, various crystallographically well defined orientations of the martensitic unit cell are possible and thus a characteristic martensitic microstructure is produced in which variants of different crystallographic orientations are connected to one another by twin boundaries. Shape memory alloys that have a reversible martensite transition can be easily deformable. Thus with the application of a mechanical compressive stress, variants with the short crystallographic axis grow parallel to the stress axis at the cost of the variants of different orientations. In materials with a reversible martensitic transition, heating into the austenite phase restores the original shape again, since in the cubic phase all axes of the unit cell are identical. With several cycles exerting mechanical compressive stresses and heating, defects in the structure can be produced, which act as nucleation sites in the martensite formation. The structure can thus be reminiscent of the unequal distribution of the variants and thus the shape memory effect is produced. In this so-called thermal “two-way shape memory effect” the temperature is used as a control parameter.
Martensitic and ferromagnetic materials have common properties. For example, both material classes have respectively one characteristic order temperature. Below the martensitic transition temperature a parallel alignment of adjacent martensitic unit cells is favored, similar to a parallel alignment of magnetic moments below the Curie temperature. Magnetic shape memory alloys have therefore two so-called “ferroic” properties. They are martensitic and ferromagnetic. Different combinations of these ferromagnetic properties can be used for actor modi.
The first actuator mode is based on the coupling between crystal structure and spontaneous magnetization. In the magnetic field the phase with the higher magnetic moment is favored in terms of energy, which permits a shift of the martensitic transition temperature TM (Vasil'ev, A. N. et al.: Phys. Usp. 46 (6), 559 (2003); Cherechukin, A. A. et al.: Phys. Lett. A 291, 175 (2001)]. As also with well trained thermal shape memory alloys, this “magnetically induced martensite” (MIM) as actuator can be operated close to the martensitic transition temperature [Kainuma, R. et al. Nature 439, 957 (2006)]. The second actuator mode was observed in single crystals within the martensite phase. The so-called “magnetically induced reorientation” (MIR) is thereby used, also referred to as magnetic shape memory effect. In MIR the difference in the total energy, with alignment of the magnetization in different crystal directions, must be observed. This is described with the magnetocrystalline anisotropy energy [O'Handley, R. C. Modern Magnetic Materials (John Wiley & Sons, Inc, New York, 2000]). The coupling between crystallographic orientation and the preferred direction of magnetization renders a change of the martensitic microstructure. A magnetic field can thus be used to move the twin boundaries such that the proportion of martensitic variants that have their energetically favorable easy magnetization axis parallel to the outer field increases. Since the lattice spacings of the martensitic unit cell are very different, this reorientation leads to a considerable change in length. MIR results in reversible, relative length changes of up to 10% [A. Sozinov, et al., Appl. Phys. Letters, Vol. 80, 1746 et seq. (2002)]. This results in advantages for the application of magnetic shape memory materials compared to the usual piezostrictive and magnetostrictive materials. An external magnetic field is used as a control parameter here. MIR itself has been described in WO 09808261 and WO 9703472. The production and use of magnetic field and strain sensors based on MIR is described in WO 03078922.
Actuators that use the principle of MIR are described in U.S. Pat. No. 6,515,382. This also discloses a valve in which a thin magnetic shape memory element is used as an active element (FIGS. 30-32).
Another approach to constructing thermally controlled microactuators from shape memory alloys is described by Kohl et al. [U.S. Pat. No. 7,142,341 A1]. Here the actuator element is heated above the Curie temperature so that there is no longer an active attraction to an additional permanent magnet. The decisive advantage of the approach claimed here is that this additional permanent magnet is no longer required to generate an external field.
Thin magnetic shape memory layers are of particular importance for microactuator applications, since the complexity of additional levers and additional mechanical components can be avoided due to the large relative strain.
Epitaxial layers are considered to be the most promising, since the largest strains so far have been achieved only in solid single crystals [Dong, J., et al.: Appl. Phys. Lett. 75 (10), 1443 (1999); Jakob, G. & Elmers, H. J.: J. Magn. Magn. Mater. 310 (2), 2779 (2007); Jakob, G., et al.: Phys. Rev. B 76 (17), 174407 (2007); Buschbeck, J., et al.: Phys. Rev. B 76, 205421 (2007)].
However, MIR has also been reported in orthorhombic, epitaxial Ni—Mn—Ga layers deposited on MgO (001) [Thomas, M. et al.: New J. Phys. 10, 023040 (2008)] and SrTiO3 [Heczko, O., et al.: Appl. Phys. Lett. 92, 072502 (2008)]. A reorientation was observed although a macroscopic extension of the layer was prevented by the substrate. A low stress at the level of a few MPa leads to the blocking of MIR [Murray, S. J., et al., J. Appl. Phys. 89 (2), 1295 (2001)]. A detachment of the magnetic shape memory layers from the rigid substrate is therefore necessary in order to achieve an actuator effect on a macroscopic scale. Dong [Dong, J. W. et al. J. Appl. Phys. 95 (5), 2593 (2004)] reports on indications of length changes by MIR in the measurement of a free-standing bridge of an epitaxial Ni—Mn—Ga layer in the magnetic field. However, the effect was observed only considerably below room temperature.
By way of the so-called sacrificial layer technology, (in part) free layers, such as are required in positioning systems (U.S. Pat. No. 6,251,298 B1, U.S. Pat. No. 6,214,244 B1) or acceleration and position sensors (JP 11103076 A1, JP 11220135 A1) can be produced. This technology permits the production of completely or partially freestanding layers without having to dissolve the substrate itself. The completely or partially freestanding layers are instead produced by selective removal of the sacrificial layer inserted between the substrate and the functional layer. Si (A. Maciossek, et al., Microelectronic Engineering, Vol. 27, 503-508, (1995), porous Si (T. E. Bell, et al., J. Micromech. Microeng., Vol. 6, 361-369 (1996); US 2006 115 919 A1), Cu (S. D. Leith, et al., J. Micromech. Microeng., Vol. 9, 97-104 (1999)), Mg (E. K. Kim, et al. Thin Solid Films, Vol. 496, 653-657 (2006)) amorphous materials, such as SiO2 (M. Lee et al., Thin Solid Films 447-448 (2004) 615-618), or also polymers, such as the polyimide (A. B. Frazier, et al., J. Microelectromech. Sys. 6 (1997) or water-soluble polyvinyl alcohol (PVA) can be used as sacrificial layers. PVA as a soluble substrate was used i.a. by Ohtsuka et al. to produce self-supporting Ni—Mn—Ga layers [Ohtsuka, M., et al., Mat. Sci. Eng. A 378 (1-2), 377 (2004)]. These layers have a strain of approx. 0.03% based on MIM. However, a subsequent heat treatment as well as a training were necessary for this.
It is relevant to the application to achieve a high relative length change which, in contrast to the absolute length change, is a length change normalized to the initial dimensions. The relative length change is often given as a percentage and is independent of the dimensions of the moveable magnetic shape memory element.
The disadvantage of the solutions of the prior art for magnetic shape memory alloys is that a magnetic field has to be generated for which complex coils systems and magnetic flux guides have to be used. This impedes in particular the integration into microsystems, since many production steps are necessary for the production of microcoils (Wurz, M. et al. IEEE Trans. Mag. 42 (2006) 2468).
It is also a disadvantage that with non-magnetic shape memory alloys, a complex thermomechanical training is required in order to achieve a reversible two-way actuator effect. In particular for integrated microsystems, this requires an additional expenditure that is usually not practical.